This invention relates broadly to aircraft automatic braking systems and more particularly to servo valves for controlling automatic braking systems which are provided with a manual override capability.
Early commercial airplanes were provided with a pair of manually controlled brake systems, one pedal controlled the braking of the wheels on the left and one pedal controlled the braking of the wheels on the right side of the aircraft. Next, anti-skid systems were added to prevent locking and skidding of the wheels. Recently, automatic braking systems were added to automatically brake the aircraft on touchdown or in the case of a rejected takeoff in an effort to maintain a constant preselected level of deceleration during the arresting roll. Deceleration is accomplished by modulation of an electro hydraulic valve in response to a comparison of the preselected deceleration rate and the actual deceleration rate as may be determined by a linear accelerometer aboard the aircraft or by measuring wheel speed.
Manual override is considered a mandatory feature of any automatic aircraft braking system. The initial systems accomplished this feature by paralleling the manual brake valve with an automatic brake valve. Weight is reduced as well as complex plumbing circuitry when the manual valve and the automatic valve are integrated into one valve as taught in U.S. Pat. No. 4,120,540 issued to Devlieg.
Devlieg teaches three alternate embodiments of the brake valve which allow the automatic and pilot operated braking pressure commands to operate in an AND, OR, or AND/OR, fashion. In the OR mode whichever command, i.e. the pilots pedal pressure or the automatic brake pressure signal, dictates the highest brake pressure controls. In the AND mode the two commands are additive. In the AND/OR mode features of both of the prior embodiments are present. FIGS. 15, 17, and 19 of the reference plot brake pressure versus pedal force for the three alternate modes. Also shown is how the curves vary with different magnitudes of automatic brake command pressure. For the sake of simplicity in the analysis and discussion, a family of parallel lines is shown representing various magnitudes of pressure which are maintained constant. It is clear from the plots that when there is an automatic brake command signal and the pilot applies force to the brake pedal there is no increase in brake pressure until the pedal force exceeds the brake force generated by the automatic brake command signal. If you were to plot pedal force versus pedal travel for the valve of FIGS. 10-13 and ignored the force of the biasing springs 311 and 312, as suggested in the reference, the curves would look just like FIGS. 15, 17, and 19. In the case of the OR valve of FIG. 15, the pedal would not move until the force reached a magnitude equal to that represented by the automatic brake command signal at which time the pedal would deflect without any further force until it exceeded the force dictated by the command signal. In the AND valve configuration of FIG. 17, there is no pedal deflection until the pedal force again reaches a level equal to the pressure generated by the automatic brake command signal, at which time it follows the particular slope determined by the level of the automatic brake command signal. A plot of pedal force versus pedal displacement for this mode would indicate that the pilot would experience a free pedal until he picked up the load. The AND/OR valve of FIG. 19 avoids the free pedal problem associated with the AND valve in that there would be no pedal movement until the pedal force exceeded the force level of the automatic brake command signal at which time it follows one of the family of parallel slopes which depend on, again, the magnitude of the automatic brake command signal. Devlieg specifically teaches that the advantages of the AND/OR valve is to optimize the pilot's "feel" of the system. However, contrary to the assumption discussed earlier, the automatic brake command signal is not a constant, but is a function of the rate of deceleration of the aircraft and may very well be changing constantly which would constantly change the particular line in the family of lines being followed. At best, as taught in Devlieg, the pilot is experiencing either a constantly changing feel, a pedal which sees no deflection until a force equal to the command force is exerted on the pedal at which time the pedal deflection is very large without an increase in force, or a combination of the two.
It is an object of the present invention to provide an aircraft brake valve, featuring both an automatic control function and a manual control function integrated into a single metering valve.
It is a further object of this invention to provide a fail-safe system that always allows the manual control function to override the automatic control function.
It is another feature of the present invention to provide, in the nomenclature terms noted above, an OR valve in that either the automatic command or the manual command, whichever generates the highest pressure, overpowers the other to exclusively control the braking pressure.
A primary feature of this invention is to provide the pilot with a brake feel force in direct proportion to the manual brake command applied, which is always the same, even though there may be a concurrent automatic brake command.